Here is the full book on line. Here it is in hard copy, which I recommend you get because it allows for better singing and planning.
Here is a portion of Richard's commentary in the foreword:
I designed this volume for choirs consisting mostly or exclusively of soprano and alto voices. The trio texture was popular among composers of the late Cecilian Movement, whose settings and transcriptions can still be heard in women’s religious communities.
The Introits selected cover the entire liturgical year for Sundays and Solemnities of the traditional Roman Missal (1962), but most are used in the modern Missal as well. Each Introit retains its Gregorian psalmtone verse, whose proper modality is reflected in the harmonized Introit itself. I have tried to preserve the natural rhythm of the Latin text, while still uncovering some type of motivic repetition. I have resorted only occasionally to text repetition when it helped to balance the musical phrase. I have made frequent use of mixed meters, without cluttering the score by marking all the metrical changes. Conductors should mark the changes, and conduct them (beating the half-note, as much as possible); singers need only keep an even quarter-note pulse, whether duple or compound, and follow the natural rhythm and accentuation of the text. Textual considerations will also govern the choice of tempo, which should always have a strong sense of forward movement.
And here is a sample page from the first Sunday in Lent.